This invention relates to tape casting, and, more particularly, to an apparatus that produces a highly uniform cast tape.
Tape casting is a technique used to prepare thin films of polymeric materials from slurries or from melts. The process is conceptually straightforward. A casting head includes a reservoir which contains a flowable material to be cast. A carrier film is transported horizontally past the bottom of the reservoir. Some of the flowable material to be cast flows with the carrier film and thence out of the reservoir. The thickness of the cast material on the carrier film is governed by the position of a portion of the wall of the reservoir, termed a xe2x80x9cdoctor bladexe2x80x9d. The doctor blade is vertically movable with a micrometer adjustment, to set the height of the opening between the bottom of the doctor blade and the top of the carrier film. The flowable material flows through this opening, whose vertical dimension determines the thickness of the cast material on the carrier film. After the carrier film and the flowable material cast onto it leave the casting head, they are heated gently to evaporate solvents and carriers from the cast material, so that it hardens into the cast tape product.
For some applications, the final cast tape must have an extremely uniform material density across the entire transverse width of the cast tape. That is, the density of a sample of the final product taken from the transverse center of the cast tape must be as nearly identical as possible to the density of a sample taken from the transverse edge of the cast tape. With existing tape casting machines, it is difficult to achieve a uniformity of density of better than about 5 percent for moderately wide tapes. For example, the weight of equal-sized samples taken from the transverse center of a 13-inch wide tape is typically about 5 percent less than that of samples taken near the transverse edge of the tape.
There is a need for an improved approach to tape casting that achieves improved uniformity of the density of the cast tape. The present invention fulfills this need, and further provides related advantages.
The present invention provides a tape casting machine and a method for its use. The tape casting machine produces a final cast tape whose transverse density is controllable according to parameters established in the tape casting machine. Most preferably, the density is uniform across the width of the cast tape, but it may instead be controllably varied.
In accordance with the invention, a tape casting machine comprises a casting base lying generally in a horizontal plane and having a feed location, and a casting head positioned to dispense a flowable material onto the casting base. The casting base and the casting head are movable relative to each other. The casting base typically comprises a support block having an upper surface, a carrier film, and a film transport that moves the carrier film across the upper surface of the support block below the casting head. The casting head comprises a reservoir containing the flowable material, and a doctor blade extending across a transverse width and having a lower margin disposed adjacent to but not contacting the casting base such that flowable material exits from the reservoir to the feed location of the casting base between the lower margin of the doctor blade and the casting base to form a cast tape. The lower margin includes a lower leading margin adjacent to the reservoir and a lower trailing margin remote from the reservoir. The lower trailing margin of the doctor blade has a curvature in at least one of the horizontal plane and a vertical plane perpendicular to the horizontal plane. Preferably, a heater is positioned downstream of the casting head to heat the tape cast onto the casting base.
In one form, the lower trailing margin of the doctor blade is curved in the horizontal plane. A width of the doctor blade between the lower leading margin and the lower trailing margin is greater at a transverse edge of the doctor blade than at a transverse centerline of the doctor blade, most preferably by an amount of from about 6.9xc3x9710xe2x88x924 to about 38xc3x9710xe2x88x924 inches per inch of transverse width.
In another form, the lower trailing margin of the doctor blade is concavely curved in the vertical plane relative to the casting base. The lower trailing margin is further from the casting base at a transverse centerline of the doctor blade than at a transverse edge of the doctor blade. A distance from the casting base to the lower trailing margin of the doctor blade in the vertical plane is greater at a transverse centerline of the doctor blade than at a transverse edge of the doctor blade, most preferably by an amount of from about 7.7xc3x9710xe2x88x926 to about 76.9xc3x9710xe2x88x926 inches per inch of transverse width.
In one embodiment, the casting base comprises a support block having an upper surface, a carrier film, and a film transport that moves the carrier film in a transport direction across the upper surface of the support block below the casting head. The casting head dispenses the slurry onto the moving carrier film. The casting head includes the doctor blade having the lower leading margin contacting the flowable material, which is preferably a slurry, in the reservoir and disposed perpendicular to the transport direction and adjacent to but not contacting the carrier film. Flowable material exits from the reservoir to the feed location of the carrier base between the lower leading margin of the doctor blade and the carrier film to form the cast tape.
The vertically and/or horizontally curved doctor blade alters the distribution of material within the cast tape. The density of the material in the cast tape may be controlled according to the curvature and geometry of the doctor blade. In particular, the curvature and geometry may be selected to produce a uniform density of the cast material across the transverse width of the cast tape. (The curvature and geometry may instead be selected to produce a controllably nonuniform density of the cast material across the transverse width of the cast tape, but this structure is of less interest than the uniform-density material.) This approach is usable with otherwise-conventional tape casting apparatus. It permits the properties of the cast tape to be controlled more precisely than heretofore possible.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.